1. Field of the Invention
The invention relates to a particle beam apparatus in which an object is irradiated with a focused beam of electrically charged particles, such as electrons, positrons, or ions, and more particularly to such a particle beam apparatus in which the particle energy can be set at values below 1 keV. Such apparatuses are used in the form of scanning electron microscopes, particularly for inspection of semiconductor wafers and also for imaging and analysis of objects with low conductivity. In order to avoid charging of the specimen surface, or for analysis of regions near the surface, the energy when incident on the object should not exceed certain limiting energies which lie between 10 eV and 5keV.
2. Discussion of Relevant Art
Since the chromatic imaging errors of the electrostatic or magnetic lenses used for focusing the particle beam are inversely proportional to the particle energy, the resolution defined by the focal diameter of the particle beam is substantially worse at low particle energies than at higher particle energies. To solve this problem, it is known to first accelerate the particles to a substantially higher energy than the desired target energy when incident on the object and to reduce the particle energy by means of a decelerating device shortly before incidence on the object. Such a decelerating device usually consists of two or more electrodes to which an electrostatic decelerating field is applied. Braking of the particles can take place either in the rear portion of the objective or after leaving the objective, between the objective and the object.
Such heretofore known solutions can be divided into two groups.
In the first group of devices, the beam guiding tube is at ground potential from the anode at least as far as to the pole piece gap of the objective, and the object is at a the anode at least as far as to the pole piece gap of the objective, and the object is at a high voltage potential, which has the same sign as the cathode potential. Such arrangements are described in the article by KJ. Plasko et al., "Low energy electron beam lithography",in Optical Engineering, March/April 1983, pages 195ff., in U.S. Pat. 5,389,787, and in European Patent EP-A2 0 769 799. At low target energies, particularly at target energies below 1 keV and an energy of the particles within the beam guiding tube of 10-20 keV, the specimen has to be placed at a correspondingly high potential of 9-19 kV. Besides insulation problems that arise at the specimen table, voltage flashovers between the object and the objective, or damage to the object due to internal currents arising from switching on and off the high voltage, can easily arise here. Detection of the secondary electrons emitted by the object is also very difficult, since these secondary electrons have to be extracted and accelerated again without affecting the primary beam.
In the second variant, the object and the electrode of the decelerating device on the object side are at ground potential, and the beam guiding tube from the anode as far as to the first electrode of the decelerating device is at a high anode potential, the sign of which is chosen such that a deceleration of the particles takes place when they leave the beam control tube. Such systems are described, for example, in German Patent DE 29 22 325 C2 and in European Patent EP 0 180 723 B1. With the arrangement according to German Patent DE 29 22 325 C2, the setting of the target energy takes place by variation of the potential of the beam guiding tube. A fixed but opposite potential between the cathode and the anode is superimposed on this variable anode potential. At target energies between 0.5 and 1 keV, the whole beam guiding tube from the anode to the objective is at a potential that is only 0.5-1 keV smaller in absolute value than the potential difference between cathode and anode. Since insulation from the surroundings has to be correspondingly set up, this arrangement can only be used at low maximum energies. For target energies below 0.2 keV, the emission becomes increasingly unstable with decreasing target energy, since the cathode potential nearly corresponds to the ground potential and therefore any stray field can affect the emission. This arrangement is therefore of little use for target energies below 0.2 keV.
In an arrangement according to European Patent EP 0 180 723 B 1, it is known from the apparatus LEO 1500 of LEO Elektronenmikroskopie GmbH, 73446 Oberkochen, Germany to keep the potential of the anode and of the beam guiding tube constant, independently of the target energy, as far as the braking device, and to vary the cathode potential in order to vary the target energy. This leads to the disadvantage that when there is a change of the target energy the energy of the particles within the beam guiding tube also changes. Consequently all the parameters within the imaging system, particularly the strength of the magnetic lenses, have to be correspondingly matched. This is especially critical at low energies, and thus low energies in the beam guiding tube, since small errors in settings already have large effects. Therefore the low energy region, particularly the region of target energies below 2 kV, has to be correspondingly finely equalized. Also, in this device, the cathode potential corresponds to the target energy, as against the ground potential, so that in this arrangement also, the emission is unstable at target energies below 0.2 keV.